Apparatus and method for reactive power control

ABSTRACT

Apparatus and method for controlling reactive power. In one embodiment, the apparatus comprises a bidirectional power converter comprising a switched mode cycloconverter for generating AC power having a desired amount of a reactive power component.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 15/001,534, filed Jan. 20, 2016, which claims thebenefit of and priority to U.S. Provisional Patent Application Ser. No.62/105,886, entitled “Power Converter Having Reactive Power Control” andfiled Jan. 21, 2015, the entire contents of each of these applicationsis herein incorporated by reference.

BACKGROUND OF THE INVENTION Field of the Invention

Embodiments of the present invention generally relate to powerconversion and, more popularly, to a power converter having reactivepower control.

Description of the Related Art

Alternative power systems such as solar, wind, and the like generallyproduce DC power that is converted to AC power for injection onto the ACpower grid. Conversion from DC power to AC power must be performed veryefficiently to enable these alternative power systems to be commerciallyviable. One form of highly efficient power converter uses acycloconverter. A cycloconverter converts a constant voltage, constantfrequency AC waveform to another AC waveform of a lower frequency bysynthesizing the output waveform from segments of the AC supply withoutan intermediate DC link. To facilitate DC to AC conversion, a DC full orhalf bridge circuit is coupled between a DC power source and thecycloconverter. The combination of the DC bridge and the cycloconverterprovides a highly efficient DC to AC power converter (also referred toas an inverter). Cycloconverters are available in single phase andthree-phase configurations. For purposes of this description, a switchedmode cycloconverter switches a cycloconverter at a frequency that ishigher than the frequency of the AC grid.

Switched mode three-phase cycloconverters were described in literatureas early as 1985. Improvements that increased efficiency includeenhanced control requirements to achieve zero volt switching (ZVS)operation and a simplification that reduced the number of switches usedin the cycloconverter by adopting a half-bridge configuration. A furtheradvance used a half-bridge cycloconverter that included aseries-resonant circuit employing a variable frequency control, where atransformer center tap was used in conjunction with an LLCseries-resonant circuit relying upon a gapped transformer core tofacilitate efficient cycloconversion.

These advances in switched mode cycloconverter circuitry made availablehighly efficient power converters for use with alternative powersystems. The widespread use of alternative power systems has raisedconcern with traditional power generation companies regarding reactivepower control for the AC power grid.

Regulations and standards (e.g., IEC 1000-3-2) have been adopted toensure that circuitry coupled to the power grid utilizes power factorcorrection techniques to ensure that the power factor at the connectionto the power grid is unity. This regulation applies to power convertersas well as power loads. For purposes of this description, power factorcorrection (PFC) is a technique used to provide harmonic correction ofnonlinear loads that ensures that the power converter couples energy tothe grid having the sinusoidal current in phase with the sinusoidalvoltage of the AC grid. A power factor of unity is used even if thepower factor of the power grid is not unity.

Power consumers coupled to the power grid can cause reactive power to bepresent on the power grid. As such, the grid power factor is no longerunity. In some instances the power generation companies require powerconsumers (e.g., large industrial power consumers) to perform reactivepower control to reduce the amount of reactive power on the power grid,i.e., the large consumers are asked to absorb the reactive power. Thepower generation utility also compensates for reactive power on the gridin an attempt to maintain a power factor of unity.

Power converters used in alternative energy systems have not beendesigned to facilitate reactive power control; these power convertersare designed for power factor control to ensure the energy that they areproducing has a power factor of unity. As alternative energy systemsbecome larger and larger, in some areas, the power they generate byalternative generator sources may dominate the power on the power gridwithout any reactive power control.

Therefore, there is a need in the art for power converters used inalternative energy systems to facilitate reactive power control.

SUMMARY OF THE INVENTION

Embodiments of the present invention generally relate to a method andapparatus for controlling reactive power substantially as shown inand/or described in connection with at least one of the figures, as setforth more completely in the claims.

Various advantages, aspects and novel features of the presentdisclosure, as well as details of an illustrated embodiment thereof,will be more fully understood from the following description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which embodiments of the present invention can beunderstood in detail, a more particular description of the invention,briefly summarized above, may be had by reference to embodiments, someof which are illustrated in the appended drawings. It is to be noted,however, that the appended drawings illustrate only typical embodimentsof this invention and are therefore not to be considered limiting of itsscope, for the invention may admit to other equally effectiveembodiments.

FIG. 1 is a block diagram of a power generation system in accordancewith embodiments of the present invention;

FIG. 2 is a set of graphs representing magnitude versus time of:voltage/current of a single phase of the AC power being created by apower converter without any reactive power control (i.e., power factorof unity), a power representation of the voltage/current, and energyflow within a power converter having no reactive power control;

FIG. 3 is a set of graphs representing magnitude versus time of:voltage/current of a single phase of the AC power being created by thepower converter having reactive power control (i.e., power factor thatis not unity); and energy flow within a power converter being used tocreate reactive power;

FIG. 4 is a phasor representation of AC power;

FIG. 5 is a block diagram of a bidirectional power converter inaccordance with an embodiment of the present invention;

FIG. 6 is block diagram of a controller for the bidirectional powerconverter of FIG. 5 ;

FIG. 7 is a schematic diagram of one embodiment of a single phase,inverter that can be used within the bidirectional power converter ofFIG. 5 ;

FIG. 8 is a schematic diagram of one embodiment of a three-phaseinverter that can be used within the bidirectional power converter ofFIG. 5 ;

FIG. 9 is a block diagram of an alternative power system utilizing thebidirectional power converter of the present invention;

FIG. 10 is a block diagram of a second type of alternative power systemutilizing the bidirectional power converter of the present invention;

FIG. 11 is a block diagram of a third type of alternative power systemutilizing the bidirectional power converter of the present invention;

FIG. 12 is a schematic diagram of one embodiment of a three-phase VArcompensator that uses a switched mode cycloconverter; and

FIG. 13 is a flow diagram of a method for controlling reactive power inaccordance with one or more embodiments of the present invention.

DETAILED DESCRIPTION

Embodiments of the present invention include a power converter havingreactive power control. More specifically, embodiments of the inventioninclude a bidirectional power converter having the capability of flowingpower into and out of a storage element within the power converter. Thepower converter comprises a DC-side bridge coupled via a resonant tankand transformer to a switch mode cycloconverter coupled to a controller.The controller is adapted to receive a power utility defined reactivepower control schedule that is implemented by the power converter. Assuch, the bidirectional power converter may create reactive power attimes and with a magnitude defined in this schedule.

FIG. 1 is a block diagram of a power generation system 100 in accordancewith embodiments of the present invention. The power generation system100 comprises a power generator 102 coupled to a bidirectional powerconverter 104. The power generator 102 (or power source) may be any formof DC power generator including, but not limited to, a wind turbine,solar panel or panels, a battery or batteries, and the like. Powergenerator 102 provides DC power to the bidirectional power converter104. The bidirectional power converter 104 produces AC power that iscoupled to an AC grid 106. To facilitate reactive power control (RPC), apower utility RPC schedule is provided to the bidirectional powerconverter 104. The utility RPC schedule, in one simple form, maycomprise a list of reactive power amounts and the time of day at whichthe reactive power is to be supplied to the AC grid 106. Typically, thelocal power generation company or utility that manages the AC grid 106provides the RPC schedule. However, in other embodiments, an RPCschedule may contain a schedule of reactive power as a function of: ACgrid voltage (Mains Voltage), inverter output power, change in AC gridvoltage, fixed value, and the like. See Common Functions for SmartInverters, Version 3, EPRI, Palo Alto, Calif.: 2013. 3002002233.

FIG. 2 is a set of graphs 200, 202, 204 respectively representingmagnitude versus time of: voltage/current of a single phase of AC powerbeing created by a power converter without any reactive power (i.e.,power factor of unity), a power representation of the voltage/current,and energy flow within a power converter having no reactive powercontrol. More specifically, graph 200 depicts the magnitude of bothvoltage 206 and current 208 created by a power converter having noreactive power control, where the AC power has a power factor of unity(no reactive power). Note that the voltage 206 and the current 208 arephase synchronized. Typically, output of the power converter issynchronized to the voltage of the AC grid to which the power converteris supplying the power. With power factor of unity, the power graph 202depicts the power fluctuating between 0 and a positive magnitude at afrequency that is twice the frequency of the AC grid voltage.

Because the input from the power generator 102 is a constant DC powerand the output power from the bidirectional power converter 104 is apulsatile AC power during DC to AC conversion, the bidirectional powerconverter 104 must buffer the input power to create the oscillating ACoutput power. Typically, this energy buffering is accomplished using astorage device such as a capacitor within the bidirectional powerconverter. Graph 204 depicts the energy flow into and out of the powerconverter's storage device (line 212) to provide the necessary energybuffering. The average power delivered by the DC source 102 isrepresented by dashed line 218. The energy 214 above the average powerline 218 represents energy being released from the storage device andthe energy 216 below the average power line 218 represents energy beingstored in the storage device. When no reactive power is needed, theenergy storage and release is synchronized at twice the frequency of thevoltage of the AC grid, i.e., synchronous with the power of the AC grid.

FIG. 3 is a set of graphs 300, 302 respectively representing magnitudeversus time of: voltage/current of a single phase of the AC power beingcreated by the power converter 104 when employing reactive power control(i.e., power factor that is not unity); and energy flow within the powerconverter 104 being used to create reactive power. More specifically,the graph 300 depicts the magnitude of both voltage 304 and current 306created by the power converter 104 when employing reactive powercontrol. Note that the voltage 304 and the current 306 are not phasesynchronized. The phase offset represents the amount of reactive powerbeing created by the power converter 104. Reactive power is 90° out ofphase with the AC grid voltage and the reactance may be lagging orleading. The ability to create reactive power requires a bidirectionalpower converter because power must be able to flow to the AC grid 106 aswell as from the AC grid 106.

The graph 302 depicts the energy flow (i.e., line 308) into and out ofthe storage device within the bidirectional power converter 104. Notethat a portion of the curve at 314 is below zero magnitude levelindicating that energy must flow from the grid 106 during this period.For magnitudes above zero, energy is flowing into the grid 106. As such,the production of reactive power is only possible with a bidirectionalpower converter.

FIG. 4 is a phasor representation 400 of AC power. Phasor 402 representsreal power of which the current is in phase with the AC grid voltage.Phasor 404 represents imaginary power (units of VAr—Volt-Amps reactive)of which the current is quadrature to the AC grid voltage. Phasor 406represents the vector addition that results if the real and imaginarypower vectors 402 and 404 are combined to obtain a reactive resultantphasor 406. This reactive phasor 406 represents the reactive power beinggenerated and this will be specified in units of VA (Volt-Amps). Theunits VA are used for reactive loads and the way of calculating VA is tosimply multiply the magnitude of the voltage by the magnitude of thecurrent in amps—ignoring the fact that voltage and current are not inphase. The reactive load (in units of VA) has two components—a realpower component (in units of VV) and an imaginary power component (inunits of VAr). This relationship can be expressed as a vector identity:VA=W+VAr (where all three entities represent vector components). Theimaginary power component is also referred to herein as a reactive powercomponent.

FIG. 5 is a block diagram of a bidirectional power converter 104 inaccordance with an embodiment of the present invention that is capableof creating reactive power. The bidirectional power converter 104comprises a controller 500 and an inverter 502. Embodiments of theinverter 502 are described in detail with respect to FIGS. 7 and 8below. The controller 500 comprises a first sampler 504, a secondsampler 508, a current sampler 526, a maximum power point tracking(MPPT) controller 506, a phase lock loop (PLL) 512, a cosine table 516,a sine table 518, a first multiplier 520, a second multiplier 522, areactive power controller (RPC) 514 and a summer 524. In some otherembodiments, one or more of the first sampler 504, the second sampler508, and the current sampler 526 may be components external to thecontroller 500.

The power controller 500 receives as input: DC power of the DC source(e.g., the first sampler 504 samples values of DC voltage and DCcurrent), AC voltage sample of the AC grid voltage (e.g., the secondsampler 508 samples values of the AC voltage), a utility RPC schedule510, and the time of day (e.g., from a “real time clock” functionresiding within the controller 500). In one embodiment, the time of dayis used in conjunction with the utility RPC schedule 510 to define thetime during the day when particular values of reactive power need to becreated and applied to the AC grid. In other embodiments, the RPCschedule 510 may contain a schedule of reactive power as a function of:AC grid voltage (mains voltage), inverter output power, change in ACgrid voltage, fixed value, and the like.

In operation, the second sampler 508 creates a digital signalrepresentative of an instantaneous voltage of the AC grid voltage. Thedigital representation is coupled to the PLL 512 as well as the RPC 514.The PLL 512 generates a phase counter signal that indexes two look uptables 516 and 518—the table that is in phase with the AC grid voltageis referred to as the sine table 518 and the table that is quadrature tothe AC grid voltage is referred to as the cosine table 516 (otherconventions could be used). The outputs from the sine and cosine tables518 and 516 represent normalized (by definition to unity—i.e., max valuefor sine=1) representations of the AC grid voltage (for sine) and thequadrature of the AC grid voltage (for cosine).

The MPPT controller 506 generates a signal Dreq and couples thegenerated Dreq signal to the first multiplier 520, while the RPC 514generates a signal Qreq and couples the generated Qreq signal to thesecond multiplier 522. The two signals Dreq and Qreq represent therequested real and imaginary currents, respectively, that are to bedelivered to the AC grid. Dreq is the direct current request—i.e., thereal current component and is supplied from the MPPT controller 506. Inorder to determine the signal Dreq, the MPPT controller 506 receives arepresentation of the DC source voltage from the sampler 504 andreceives a representation of the current from the DC source from acurrent sampler 526 coupled between the DC source 102 and the MPPTcontroller 506. The MPPT controller 506 operates in a well-known mannerknown to those skilled in the art to derive a value Dreq for the desiredreal output current, while maintaining the DC source 102 operating at amaximum power point. The signal Qreq is the quadrature currentrequest—i.e., the imaginary current request and is supplied from analgorithm that is, for example, ultimately specified by the powerutility company. Typically, this algorithm, performed by the reactivepower controller 514, would adjust the requested imaginary current as afunction of the AC grid voltage as this will help regulate the voltageon the AC grid.

The signals Dreq and Qreq scale the outputs from the sine and cosinetables 518 and 516 respectively using a multiplication operation(x)—i.e., multipliers 520 and 522 respectively. The results of these twomultiplications are summed together with an addition operation (+)(summer 524) and the output becomes the Ireq signal, where Ireq is avector representation of the desired current to be supplied to the ACgrid. Ireq has a polarity that mirrors the polarity of the AC currentflowing out of the power converter 104. When the instantaneous ACvoltage and current are of the same polarity, the condition is referredto as forward power flow and, when the instantaneous AC voltage andcurrent are of opposite polarity, the condition is referred to asreverse power flow. There are different conventions for assigningpolarity to the AC voltage, AC current, and power flow direction anddefining polarity differently will result in a different but equallyvalid alternate convention.

FIG. 6 is a block diagram of one embodiment of a controller 600 for theembodiment of a bidirectional power converter of FIG. 5 (the controller600 is one embodiment of the controller 500). The control functionsdefined above with respect FIG. 5 can be implemented in hardware,software, or a combination of hardware and software. Inputs to thecontroller 600 include a digital representation of the DC sourcevoltage, a digital representation of the DC source current and a digitalrepresentation of the AC grid voltage. These signals are created usinghardware to sample and digitize the voltages and current, e.g., ananalog to digital (A/D) converter (not shown), (although in otherembodiments the controller 600 may comprise one or more modules orcomponents for sampling DC voltage, DC current, and/or AC voltage andgenerating digital representations thereof).

The controller 600 comprises a central processing unit (CPU) 602 coupledto each of support circuits 604 and memory 606; in some embodiments, theCPU 602 may further be coupled to a transceiver for communication to andfrom the power converter 102 (e.g., using power line communications).The CPU 602 may be any commercially available processor, microprocessoror microcontroller, or combinations thereof, configured to executenon-transient software instructions to perform various tasks such asthose described herein. In some embodiments, the CPU 602 may be amicrocontroller comprising internal memory for storing controllerfirmware that, when executed, provides the controller functionality. Thecontroller 600 may be implemented using a general purpose computer that,when executing particular software, becomes a specific purpose computerfor performing various embodiments of the present invention.

The support circuit 604 may include, but are not limited to, suchcircuits as power supplies, cache memory, clock circuits, and the like.The memory 606 may include read-only memory and/or random access memorythat stores data and software instructions to be utilized by the CPU602.

The memory 606 stores an operating system (OS) 620 (when needed) of thecontroller 600, where the OS 620 may be one of a number of commerciallyavailable operating systems such as, but not limited to, Linux,Real-Time Operating System (RTOS), and the like. The memory 606 storesnon-transient processor-executable instructions and/or data that may beexecuted by and/or used by the CPU 602. These processor-executableinstructions may comprise firmware, software, and the like, or somecombination thereof. In the embodiment described with respect to FIG. 6, the memory 606 comprises a sine table 608, a cosine table 610, an RPCschedule 612, and inverter control software 614. The inverter controlsoftware includes MPPT control software 616 and RPC control software618. When executed by the CPU 602, the MPPT control software 616functions as the MPPT controller 506 and the RPC control software 618functions as the RPC controller 514. The controller 600 may also beimplemented as an application specific integrated circuit (ASIC) that isspecifically programmed to perform the operations described herein(e.g., with respect to FIG. 5 ).

FIG. 7 is a schematic diagram of one embodiment of a single phaseinverter 700 that can be used within the bidirectional power converter104 of FIG. 5 (i.e., the inverter 700 is one embodiment of the inverter502). The inverter 700 comprises a storage device 702 (e.g., acapacitor), a DC full bridge 704, a resonant circuit 706, and isolationtransformer 708, and a switched mode cycloconverter 712. As describedabove, the storage device 702, coupled across the input to the inverter700, stores energy to facilitate both creation of AC power from DC poweras well as facilitating creating reactive power. The bridge 704 convertsDC power into a high-frequency AC signal. The DC bridge 704 is coupledto the resonant circuit 706 that has a resonance that is commensuratewith the high-frequency AC signal.

The AC signal is coupled to the cycloconverter 712 via the isolationtransformer 708. The cycloconverter 712 converts the high-frequency ACsignal into a signal having a power profile commensurate with the ACpower on the AC grid. The controller 500 of FIG. 5 or the controller 600of FIG. 6 is used to control the timing of the switches within thebridge 704 and the cycloconverter 712 to achieve single-phase AC powercontaining reactive power.

To reiterate, the storage device 702 operates to buffer energy duringthe power conversion process. In addition, when the bidirectional powerconverter 104 must flow power from the grid 106 to facilitate reactivepower generation, the storage device 702 stores the necessary energy.

Generally, a cycloconverter converts an AC signal of a particularvoltage/current, frequency, and phase order directly to a differentvoltage/current and/or frequency and/or phase order without the use ofan intermediate DC bus or DC energy storage. Although a single-phasecycloconverter is depicted in FIG. 7 , a cycloconverter can have asingle-phase, three-phase, or any general polyphase input. Likewise, acycloconverter can have a single-phase, three-phase or any generalpolyphase output. Accordingly, cycloconverters can be used to convertfrom a polyphase system of any order (n=1,2,3,4 . . . ) to any otherpolyphase system order (n=1,2,3,4 . . . ). Cycloconverters rely onbidirectional switches. These switches are sometimes referred to asfour-quadrant switches as they can handle voltage and current of anypolarity (i.e., the four quadrants (++, +−, −+, −−). Four-quadrant orbidirectional switches can be made by connecting two unidirectionalswitches in series such that the two switches are orientated such thatthey conduct the same current in opposite directions. Alternatively,bidirectional switches can be facilitated using a bridge rectifier and asingle unidirectional switch.

FIG. 8 is a schematic diagram of an embodiment of a three-phase inverter800 that can be used within the bidirectional power converter 104 ofFIG. 5 (i.e., the inverter 800 is one embodiment of the inverter 502).The inverter 800 comprises a storage device 802 (e.g., a capacitor), aDC full bridge 804, a resonant circuit 808, an isolation transformer806, and a switched mode cycloconverter 810. As described above, thestorage device 802 stores energy to facilitate both creation of AC powerfrom DC power as well as facilitating creating reactive power. The DCbridge 804 converts DC power into a high-frequency AC signal. The DCbridge 804 is coupled to the isolation transformer 806 via a resonantcircuit 808 that has a resonance that is commensurate with thehigh-frequency AC signal. The AC signal is coupled to the cycloconverter810. The cycloconverter 810 converts the high-frequency AC signal into asignal having a power profile commensurate with the three-phase AC poweron the AC grid. The controller 500 of FIG. 5 or the controller 600 ofFIG. 6 is used to control the timing of the switches within the bridge804 and the cycloconverter 810 to achieve three-phase AC powercontaining reactive power.

Further information can be found on cycloconverter operation in commonlyassigned U.S. application publication number 2012/0170341, published onJul. 5, 2012 having a title of “Method and Apparatus for Resonant PowerConversion” and herein incorporated by reference in its entirety.

FIG. 9 is a block diagram of one embodiment of an alternative powersystem 900 utilizing the bidirectional power converter 104 of thepresent invention to produce AC power from DC power, where the AC powerincludes reactive power. The system 900 comprises a plurality of powersources (PS) 902 ₁, 902 ₂ . . . 902 _(n), collectively referred to aspower sources 902, coupled to the bidirectional power converter 104. Thestring of power sources 902 combines to provide DC power to thebidirectional power converter 104 for conversion to AC power havingreactive power as described herein. The power sources 902 may bearranged in a large array coupled to a single bidirectional powerconverter 104 (as depicted in FIG. 9 ) or alternatively to a smallnumber of bidirectional power converters 104.

FIG. 10 is a block diagram of one embodiment of a second type ofalternative power system 1000 utilizing the bidirectional powerconverter 104 of the present invention to create AC power with reactivepower. The system 1000 comprises a plurality of power sources (PS) 1002₁, 1002 ₂ . . . 1002 _(n), collectively referred to as power sources1002, each coupled to an associated bidirectional power converter 104 ₁,104 ₂ . . . 104 _(n), collectively referred to as power converters 104.Each power source 1002 provides its DC power output to a correspondingbidirectional power converter 104 for conversion to AC power havingreactive power as described herein. The output AC power from the powerconverters 104 is coupled to a bus 1006. The power sources 1002 andtheir associated bidirectional power converters 104 may be arranged in alarge array. The AC output power is coupled to the AC power grid 106.

FIG. 11 is a block diagram of one embodiment of a third type ofalternative power system 1100 utilizing the bidirectional powerconverter 104 of the present invention to produce AC power with reactivepower. The system 1100 comprises a plurality of power sources (PS) 1102₁, 1102 ₂ . . . 1102 _(n), collectively referred to as power sources1102, coupled to a DC power aggregator 1104 such that the DC power fromthe power sources 1102 is combined in the aggregator 1104 to ahigh-voltage DC power. A multitude of DC power sources (such as powersources 1106 ₁, 1106 ₂ . . . 1106 _(n), collectively referred to aspower sources 1106) and additional aggregators (such as the aggregator1108 coupled to the power sources 1106) can be used to form a largepower array. The outputs of the aggregators 1104 and 1108 are coupled toa high-voltage DC bus 1112 that is coupled to a bidirectional powerconverter 104. The strings of power sources 1102 and 1106 and theaggregators 1104 and 1108 combine to provide DC power to thebidirectional power converter 104 for conversion to AC power havingreactive power as described herein.

FIGS. 9, 10 and 11 are intended to show a sample of the types ofalternative power system arrangements in which the bidirectional powerconverter 104 may find use. The three systems 900, 1000, and 1100 arenot meant to be exhaustive. The bidirectional power converter 104 havingreactive power control may find use in any power system where, forexample, a utility desires to control the reactive power on the AC grid106. The utility RPC schedule 108 may be coupled to the bidirectionalpower converter 104 via wired and/or wireless communication techniques,such as Ethernet, wireless techniques based on standards such as IEEE802.11, Zigbee, Z-wave, or the like, power line communications, and thelike. In some embodiments, the utility RPC schedule 108 may be manuallyentered or pre-programmed into the bidirectional power converter 104.

FIG. 12 is a schematic diagram of an embodiment of a three-phase staticVAr compensator 1200 that uses a switched mode cycloconverter 1200. Thestatic VAr compensator 1200 comprises a storage device 1204 (e.g., aresonant tank comprising a capacitor 1206 and an inductor 1208) and theswitched mode cycloconverter 1202. A static VAr compensator is one typeof bidirectional power converter, and the static VAr compensator 1200may be used as part of the bidirectional power converter 104 in one ormore embodiments.

The storage device 1204 stores and releases energy to facilitatecreating reactive power. The cycloconverter 1202 couples energy to andfrom the AC grid 106 such that the AC current is quadrature to the ACmains voltage, with a magnitude that is commensurate with the desiredamount of reactive power. The controller 500 of FIG. 5 or the controller600 of FIG. 6 may be used as the VAr compensator controller to controlthe timing of the switches within the cycloconverter 1202 to achievethree-phase AC power containing reactive power. Since the VArcompensator 1200 is not coupled to a DC source, there is no need for thecontroller to create the variable Dreq that is generated by the MPPTcontroller 506. As such, the VAr compensator controller does not includeor has deactivated the MPPT controller 516/MPPT control software 616 andthe associated sine table 518/sine table 608. In the VAr compensator1200, the signal Dreq is unnecessary because the static VAr compensator1200 is only capable of providing or consuming VAr, i.e., no real poweris generated or consumed and there is only a reactive power component tothe AC power.

FIG. 13 is a flow diagram of a method 1300 for controlling reactivepower in accordance with one or more embodiments of the presentinvention. The method 1300 is implemented using a bidirectional powerconverter having a switched mode cycloconverter (e.g., the bidirectionalpower converter 104). The bidirectional power converter is coupled to anAC line or grid, such as a commercial AC grid. In some embodiments ofthe method 1300, the bidirectional power converter is coupled to arenewable energy source (such as one or more photovoltaic (PV) modules)for receiving DC power that is converted to AC power. In one or morealternative embodiments, the bidirectional power converter is a staticVAr compensator.

The method 1300 starts at step 1302 and proceeds to step 1304. At step1304, a desired amount of a reactive power component to be generated bythe bidirectional power converter is determined. In some embodiments,the desired amount of the reactive power component may be determinedbased on a reactive power control (RPC) schedule. The RPC schedule maybe communicated to the bidirectional power converter using wired (e.g.,power line communications) and/or wireless communication techniques. Incertain alternative embodiments, the RPC schedule may be manuallyentered into the bidirectional power converter (e.g., through a webbrowser interface); in other alternative embodiments, the RPC schedulemay be preprogrammed into the bidirectional power converter.

In order to facilitate reactive power control, the RPC schedule maycomprise a list of reactive power amounts and the time of day at whichthe listed reactive power amounts are to be supplied to the AC grid.Additionally or alternatively, the RPC schedule may contain a scheduleof reactive power to be generated as a function of one or more of the ACgrid voltage, the power converter output power, a change in AC gridvoltage, a fixed value, and the like.

Typically, the RPC schedule is provided by the local power generationcompany or utility that manages the AC grid to which the bidirectionalpower converter is coupled, although in some alternative embodiments theRPC schedule may be obtained from a different source.

The method 1300 proceeds from step 1304 to step 1306. At step 1306 thebidirectional power converter generates AC power having the desiredamount of the reactive power component determined in step 1304. Thebidirectional power converter generates the AC power using its switchedmode cycloconverter as described above (e.g., with respect to FIG. 5 ).In some embodiments, the bidirectional power converter generatessingle-phase AC power; in other embodiments, the bidirectional powerconverter generates three-phase AC power.

The method 1300 proceeds to step 1308, where a determination is madewhether to continue. If the result of the determination is yes, themethod 1300 returns to step 1304; if the result of the determination isno, the method 1300 proceeds to step 1310 where it ends.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

The invention claimed is:
 1. A bidirectional power converter comprising:a storage device for storing energy; a DC bridge, coupled to the storagedevice, for producing a high-frequency AC signal; a resonant circuit,coupled to the DC bridge, having a resonant frequency proximate afrequency of the high-frequency AC signal; a transformer coupled to theresonant circuit; a switched mode cycloconverter, coupled to thetransformer, for converting the high-frequency AC signal to alow-frequency AC signal; and a controller, coupled to the DC bridge andthe switched mode cycloconverter, for controlling the DC bridge and theswitched mode cycloconverter to convert DC power from a DC power sourcecoupled to the storage device into AC power at an output of the switchedmode cycloconverter, wherein the AC power has a desired amount ofreactive power component as determined by a reactive power controlschedule comprising a schedule of reactive power to be generated as afunction of one or more of AC grid voltage, bidirectional powerconverter output power, or change in AC grid voltage.
 2. Thebidirectional power converter of claim 1, wherein the bidirectionalpower converter is a resonant converter.
 3. The bidirectional powerconverter of claim 2, wherein the reactive power control schedulecomprises a list of reactive power amounts and a corresponding time ofday for generating each reactive power amount of the list of reactivepower amounts.
 4. The bidirectional power converter of claim 2, whereinthe schedule of reactive power is further generated as a function of afixed value.
 5. The bidirectional power converter of claim 1, whereinthe bidirectional power converter is a static VAr compensator.
 6. Thebidirectional power converter of claim 1, wherein the switched modecycloconverter is one of a single-phase cycloconverter or a three-phasecycloconverter.
 7. The bidirectional power converter of claim 1, whereinthe DC power source is a battery.
 8. A method of controlling reactivepower comprising: storing energy using a storage device; producing ahigh-frequency AC signal using a DC bridge coupled to the storagedevice; converting the high-frequency AC signal to a low-frequency ACsignal using a switched mode cycloconverter coupled to a transformer;and controlling, using a controller coupled to the DC bridge and theswitched mode cycloconverter, the DC bridge and the switched modecycloconverter to convert DC power from a DC power source coupled to thestorage device into AC power at an output of the switched modecycloconverter, wherein the AC power has a desired amount of reactivepower component as determined by a reactive power control schedulecomprising a schedule of reactive power to be generated as a function ofone or more of AC grid voltage, bidirectional power converter outputpower, or change in AC grid voltage.
 9. The method of claim 8, furthercomprising receiving the reactive power control schedule.
 10. The methodof claim 9, wherein the reactive power control schedule comprises a listof reactive power amounts and a corresponding time of day for generatingeach reactive power amount of the list of reactive power amounts. 11.The method of claim 9, wherein the schedule of reactive power is furthergenerated as a function of a fixed value.
 12. The method of claim 8,wherein the switched mode cycloconverter is part of a bidirectionalpower converter, and wherein the bidirectional power converter is astatic VAr compensator.
 13. The method of claim 8, wherein the switchedmode cycloconverter is one of a single-phase cycloconverter or athree-phase cycloconverter.
 14. The method of claim 8, wherein the DCpower source is a battery.
 15. A system for controlling reactive power,comprising: a DC power source for generating DC power; and abidirectional power converter coupled to the DC power source forreceiving the DC power and comprising: a storage device for storingenergy; a DC bridge, coupled to the storage device, for producing ahigh-frequency AC signal; a resonant circuit, coupled to the DC bridge,having a resonant frequency proximate a frequency of the high-frequencyAC signal; a transformer coupled to the resonant circuit; a switchedmode cycloconverter, coupled to the transformer, for converting thehigh-frequency AC signal to a low-frequency AC signal; and a controller,coupled to the DC bridge and the switched mode cycloconverter, forcontrolling the DC bridge and the switched mode cycloconverter toconvert DC power from the DC power source coupled to the storage deviceinto AC power at an output of the switched mode cycloconverter, whereinthe AC power has a desired amount of reactive power component asdetermined by a reactive power control schedule comprising a schedule ofreactive power to be generated as a function of one or more of AC gridvoltage, bidirectional power converter output power, or change in ACgrid voltage.
 16. The system of claim 15, wherein the bidirectionalpower converter is a resonant converter.
 17. The system of claim 16,wherein the reactive power control schedule comprises a list of reactivepower amounts and a corresponding time of day for generating eachreactive power amount of the list of reactive power amounts.
 18. Thesystem of claim 16, wherein the schedule of reactive power is furthergenerated as a function of a fixed value.
 19. The system of claim 15,wherein the switched mode cycloconverter is one of a single-phasecycloconverter or a three-phase cycloconverter.
 20. The system of claim15, wherein the DC power source is a battery.